1. Field of the Invention
The present invention pertains generally to photovoltaic system charge controllers, particularly photovoltaic system charge controllers that employ maximum power point tracking, and to power electronics employed in photovoltaic system charge controllers and inverters.
2. Brief Discussion of the Related Art
Photovoltaic (PV) systems that produce electricity from solar energy have established themselves as a successful and reliable option for electrical power generation. Photovoltaic systems have continually been gaining in popularity as the cost of such systems has been reduced, as the cost of utility-supplied power has escalated, and as greater attention has been paid to the need for renewable, alternative energy sources. Basically, a photovoltaic system includes a photovoltaic (PV) array made up of one or more PV panels or modules composed of photovoltaic cells capable of converting solar energy into direct current (DC) electrical energy, a battery bank made up of one or more batteries for storing the electrical energy produced by the photovoltaic array, and a charge controller for controlling the charging of the one or more batteries with the electrical energy produced by the photovoltaic array. The direct current (DC) electrical energy produced by the photovoltaic array and/or stored in the battery bank is available to power a load. In some systems, the load may include an inverter used to convert the direct current (DC) electrical energy into alternating current (AC) electrical energy suitable to power AC loads. Photovoltaic systems are frequently employed to power loads independently of utility power, such as where electrical power from a public utility grid is unavailable or not feasible, and these photovoltaic systems are commonly referred to as “off-grid” and “stand-alone” photovoltaic systems. In other instances, photovoltaic systems known as “on-grid” and “grid-connected” photovoltaic systems are employed to supply electrical power to the public utility grid.
Photovoltaic systems have been designed with traditional charge controllers that do not employ maximum power point tracking (MPPT), and such controllers may be referred to as non-MPPT charge controllers. Non-MPPT charge controllers connect the PV array directly to the battery bank for charging. Usually there is a mismatch between the output voltage of the PV array and the voltage required to charge the battery bank that results in under-utilization of the maximum power output from the PV array. The reason for the mismatch is that most PV modules are rated to produce a nominal 12V under standard test conditions but, because they are designed for worse than standard test conditions, in actual fact they produce significantly more voltage. On the other hand, a nominal 12V battery for example requires close to an actual 12V (14V typically) depending on battery state of charge. When a non-MPPT charge controller is charging the battery bank, the PV module is frequently forced to operate at a battery voltage that is less than the optimal operating voltage at which the PV module is capable of producing its maximum power. Hence, non-MPPT charge controllers artificially limit power production to a sub-optimal level by constraining the PV array from operating at maximum output power.
A maximum power point tracking (MPPT) charge controller addresses the aforesaid disadvantage of non-MPPT charge controllers by managing the voltage mismatch between the PV array and the battery bank through the use of power electronics. The primary functions performed by MPPT charge controllers involve measuring the PV module output to find the maximum power voltage (Vmp), i.e. the voltage at which the PV module is able to produce maximum power, and operating the PV module at the maximum power voltage to extract or harvest full power (watts) from the PV array, regardless of the present battery voltage (VB).
Photovoltaic modules are made up of photovoltaic (PV) cells that have a single operating point where the values of the current (I) and voltage (V) of the cell result in a maximum power output. The maximum power voltage Vmp varies with operating conditions including weather, sunlight intensity, shading, and PV cell temperature. As the maximum power voltage Vmp of the PV module varies, the MPPT charge controller “tracks” the Vmp and adjusts the ratio between the maximum power voltage and the current delivered to the battery in order to match what the battery requires. The MPPT charge controller utilizes a control circuit or logic to search for the maximum power output operating point and employs power electronics to extract the maximum power available from a PV module.
A MPPT charge controller employs power electronics that have a higher input voltage than output voltage, hence Vmp>VB. In inverters the input voltage could be lower than the output voltage. The power electronics are conventionally designed to include a high frequency DC to DC converter that receives the maximum power voltage from the PV array as converter input and converts the maximum power voltage to battery voltage as converter output. An increase in battery charge current is realized by harvesting PV module power that would be left unharvested using a non-MPPT charge controller. As the maximum power voltage varies, the actual charge current increase that is realized will likewise vary. Generally speaking, the greater the mismatch or disparity between the PV array maximum power voltage and the battery voltage, the greater the charge current increase will be. The charge current increase will ordinarily be greater in cooler temperatures because the available power output and the maximum power voltage of the PV module increase as the photovoltaic cell temperature decreases. In addition, lower battery voltage, as in the case of a highly discharged battery, will result in a greater charge current increase.
Most MPPT charge controllers utilize power electronics designed to include a “buck” converter having topology to “buck” a higher input voltage to a lower output voltage. Buck converters, also known as “step-down” converters, are familiar in the field of power electronics and essentially include an inductor and two complementary switches to achieve unidirectional or bidirectional power flow from input to output. A first of the switches is ordinarily a controlled switch such as a MOSFET or transistor, and the second of the switches is ordinarily an uncontrolled switch such as a diode. The buck converter alternates between connecting the inductor to the input voltage (VA) from the PV array to store energy in the inductor and discharging the inductor into the battery bank. When the first switch is turned “on” for a time duration, the second switch becomes reverse biased and the inductor is connected to the input voltage VA. There is a positive voltage (VL) across the inductor equal to the input voltage VA minus the output voltage (VB), hence VL=VA−VB and there is an increase in the inductor current (IL). In this “on” state, energy is stored in the inductor. When the first switch is turned “off”, inductor current IL continues to flow due to the inductor energy storage, resulting in a negative voltage across the inductor (VL=−VB). The inductor current now flows through the second switch, which is forward biased, and current IL through the inductor decreases. In this “off” state, energy continues to be delivered to the output until the first switch is again turned on to begin another on-off cycle. The buck converter is operated in continuous conduction mode (CCM) when the current through the inductor never falls to zero during the commutation cycle. If the buck converter is operated in continuous conduction mode, the output voltage (VB) is equal to VA×d, where d is the duty cycle (d=[O,1]) of the switches. The buck converter is operated in discontinuous conduction mode (DCM) when the current through the inductor goes to zero every commutation cycle.
When a buck converter operates in continuous conduction mode, current in the inductor does not decay to zero from the previous cycle. When a metal oxide semiconductor field effect transistor (MOSFET) is used as the first switch and a diode is used as the second switch in the buck converter, the diode still has current in it when the MOSFET is turned “on”. When the diode is thusly switched from its “on”, forward biased or conducting state to its “off”, reverse biased or blocking state, the current in the diode must first be discharged before the diode blocks reverse current. This discharge takes a finite amount of time known as the reverse recovery time (Trr) of the diode. During this time, the diode may briefly conduct current backwards. Diodes that have relatively fast or short reverse recovery times may be referred to as “high speed” or “fast” diodes, while diodes that have relatively slow or long reverse recovery times are typically referred to as “low speed” or “slow” diodes. Some high speed diodes are optimized to have an “ultra-fast” Trr and these are known as “ultra-fast” diodes. High speed diodes are advantageous for providing rapid switching action with lower switching losses and lower electromagnetic interference (EMI) but are disadvantageous for their higher forward voltage, i.e. the forward voltage drop across the diode terminals that occurs when current flows through the diode in its “on”, forward biased or conducting state, which causes higher conduction losses with resultant loss of efficiency. Conduction losses can be lowered for increased efficiency by using slower diodes, but slower diodes are disadvantageous for their higher switching losses and higher EMI. Hence, buck converter design often involves a design trade-off between a slower diode (lower conduction losses but higher switching losses and EMI) and a high speed diode (high conduction losses but lower switching losses and EMI).
The efficiency of buck converters can be improved using a technique known as “synchronous rectification”. In synchronous rectification, the diode that serves as the second switch in the buck converter can be replaced with a power MOSFET which, like all power MOSFETs, has an intrinsic or inherent anti-parallel parasitic body diode between the source and the drain of the MOSFET transistor. When the body diode is forward biased and conducting current, the MOSFET transistor is turned “on” a short time after the body diode has started to conduct. The MOSFET transistor is turned “off” a short time before the first switch in the buck converter is going to turn back “on”. The MOSFET of the second switch in the “on” state behaves as a low value resistance, reducing the forward voltage and yielding lower losses. While this MOSFET is “on”, the forward voltage drop of the body diode is limited to the “on” resistance of the MOSFET transistor. This forward voltage drop can be significantly lower than the voltage drop in the diode referred to above as the second switch in the buck converter, thereby lowering conduction losses. However, the synchronous rectification approach is disadvantageous because the anti-parallel body diode of the MOSFET typically has a very long reverse recovery time resulting in higher switching losses and higher EMI.
Another approach to reducing conduction losses in buck converters involves the use of Schottky diodes, which have a characteristically low forward voltage drop and fast switching action. In low voltage buck converters, a Schottky diode can be placed in parallel with a MOSFET and its intrinsic body diode to serve as the second switch. Because the forward voltage drop of the Schottky diode is much lower than the forward voltage drop of the body diode, most of the current that needs to be conducted will flow through the Schottky diode, which has a very short reverse recovery time. Schottky diodes are unavailable for higher voltages, i.e. greater than 150 volts, thus eliminating them as an option for higher voltage buck converters. Even for applications requiring voltages in the range of 80-100 volts, the forward voltage drop of Schottky diodes increases to be comparable or greater than the forward voltage drop of the MOSFET body diodes such that the aforementioned design approach becomes unworkable.
Buck converters can be designed with the second switch composed of two MOSFETs placed in series with a parallel diode, which can be any suitable fast diode including Schottky diodes and ultra-fast diodes. The intrinsic body diodes of the MOSFETs connected in series have twice the forward voltage when “on”, so that the parallel diode is favored up to its limitations. A drawback to this approach is that conduction losses are increased because the current must flow through two FETs when in synchronous rectification mode or when carrying current in the forward direction depending upon the topology of the buck converter. Further disadvantages pertain to the floating source on one of the MOSFETs and the inability for the gates of the transistors to be connected and driven from the same gate drive, which may arise when the MOSFETs are different part numbers or due to different part tolerances even when the MOSFETs are the same part number. Although an additional diode can be incorporated in the design to allow the gates to be driven from one common gate drive, the additional diode gives rise to disadvantages including increased forward voltage for diodes having lower reverse recovery times, the effect of leakage inductances, the rate at which current switches to/from the additional diode, slower current slew rates, longer dead times, and conduction losses.
Typically FETs such as MOSFETs are arranged in electrical circuits with the FETs connected in a “source to drain” series configuration. Electrical circuits in which two FETs are connected in a “back to back” series configuration have thus far been limited to applications involving solid state relays and matrix converters. Commercial solid state relays that utilize a “back to back” FETs series configuration, without a parallel diode, block bidirectional DC and AC voltages. The positive and negative polarities blocked in such relays are identical, and therefore the FETs used are identical. The FETs carry current in either direction when “on” and block voltage in either direction when “off”. Matrix converters, which are commonly found in large motor drives, are most often used to convert multi-phase AC of one frequency and voltage to multi-phase AC of another voltage and frequency without the need for first converting AC input voltage to DC voltage, filtering with a capacitor, and running through an inverter to generate AC output at a lower voltage and different frequency. Each switch in a matrix converter must block symmetric positive and negative voltages because at any moment in time some of the AC input phases are negative while the AC output needs to be positive, and vice versa. Most matrix converters are used at high voltages, and the MOSFETs needed for these high voltages have a very high resistance. Consequently, most matrix converters employ insulated gate bipolar transistors (IGBTs), which are generally less expensive than MOSFETs for the same level of efficiency.
It is seen from the above that the need exists for a buck converter for a maximum power point tracking charge controller that overcomes the aforementioned problems and disadvantages while allowing the use of MOSFETs that do not have to be of the same type or rating, allowing the use of lower voltage MOSFETs, allowing the use of MOSFETs having a smaller die area, allowing the gates of the MOSFETs to be driven from the same gate drive, allowing simplification of the gate drive, allowing the use of any suitable diode including ultra-fast diodes to optimize for low switching losses, allowing the use of diodes that would otherwise be unsuitable because of their high forward voltage, minimizing the impact from leakage inductance, achieving faster current transfer, minimizing switching losses and dead time, minimizing conduction losses, providing greater overall efficiency, and reducing cost. The need further exists for an electrical switch configuration that overcomes the aforementioned problems and disadvantages in buck converters while being similarly advantageous for use in boost or other types of converters as well as inverters.